This invention relates generally to electrical and transformer oils and more particularly to a method for reducing the gassing tendency of paraffinic and naphthenic based electrical oils or mixtures of same.
Electrical insulating oils are formulated so that they may meet or exceed certain specific, performance conditions. These conditions include a minimum pour point, a maximum kinematic viscosity and enumerated limits on interfacial tension, impulse breakdown strength, gassing tendency and levels of acid number and sludge produced in oxidation tests.
Current commercial practice is to use naphthenic distillates as the basestock for electrical insulating oils. Typically the basestock is combined with an effective amount of an antioxidant additive, commonly a hindered phenol. Examples of such antioxidants include 2,6-di-t-butyl phenol and 2,6-di-t-butyl cresol. Use of oxidation inhibitors, however, is limited. ASTM D3487 describes Type I oils as being restricted to a maximum of 0.08 wt % oxidation inhibitor and Type II oils, 0.3 wt % inhibitor.
As electrical equipment manufacturers develop more efficient electrical devices there will be a need for electrical oils that have better oxidation resistance than the current oils based on naphthenic basestocks. Higher oxidation resistance can be achieved by use of paraffinic basestocks; however, paraffinic basestock exhibit what is referred to as a positive gassing. The gassing tendency of an oil is a measure of the rate at which hydrogen gas is either evolved or absorbed in an insulating medium when that medium is subjected to electrical stress sufficient to cause ionization. A positive gassing tendency indicates that hydrogen gas is given off, while a negative gassing tendency indicates that hydrogen gas is absorbed. A negative gassing tendency, or very low positive tendency, is desirable since it will minimize the build-up of hydrogen gas which could react with oxygen in the presence of a discharge spark to cause an explosion in the electrical device. Insulating oils shown to have gas absorbing characteristics have been used to advantage in reducing equipment failure, particularly in cables and capacitors.
The gassing tendency of electrical oils is measured by test method ASTM D 2300. Oils that evolve hydrogen gas have a positive test value and those that absorb hydrogen gas have a negative test value.
The present invention is based on the discovery that a blend of certain hindered phenols is capable of reducing the gassing tendency of paraffinic or naphthenic basestocks.
Thus, in one embodiment there is provided an electrical oil comprising a major amount of a paraffinic or naphthenic basestock having a viscosity greater than about 5 cSt @ 40xc2x0 C., preferably 5 cSt to 1000 cSt @ 40xc2x0 C., and a minor amount of at least one hindered phenol represented by formula I and at least one hindered phenol represented by formula II 
where R1 and R2 may be the same or different alkyl groups, especially branched alkyl groups, containing 3 to about 9 carbon atoms and wherein the weight ratio of phenols of formula I and II is in the range of 1:10 to 10:1. Preferably the phenols of formula I and II are present in an amount greater than about 0.5 wt % based on the weight of the oil.
In another embodiment, the composition of the invention includes a minor amount of a tolyltriazole metal deactivator.
In yet another embodiment, a method for reducing the gassing tendency of a paraffinic or naphthenic electrical oil comprises adding to the oil at least one phenol of formula I and at least one phenol of formula II in the weight ratio of 1:10 to 10:1, the combined phenols being greater than about 0.5 wt % based on the weight of the oil.
The composition of the present invention utilizes a major amount of a paraffinic or naphthenic oil with a viscosity greater than about 5 cSt at 40xc2x0 C. and preferably from about 5 cSt to about 1000 cSt at 40xc2x0 C. Especially preferred is a paraffinic oil. Typically the paraffinic oil is one having a viscosity of about 25 cSt to 150 cSt at 40xc2x0 C. In general the oil also will have a pour point in the range of about xe2x88x9250xc2x0 C. to about xe2x88x9215xc2x0 C. Examples of such oils are solvent refined paraffinic basestocks with a viscosity of about 30 cSt at 40xc2x0 C. such as Solvent Neutral 145 and a hydrotreated paraffinic base stock with a viscosity of about 25 cSt at 40xc2x0 C. such as EHC 45, both of which are sold by Exxon Corporation, Irvine, Tex.
The composition of the present invention also includes at least one phenol represented by formula I 
and at least one phenol represented by formula II 
where R1 and R2 may be the same or different alkyl groups, especially branched alkyl groups, containing 3 to about 9 carbon atoms. Preferably in the above phenols R1 and R2 are tertiary butyl groups.
The amount of the combined phenols of formula I and II are generally present in the composition in a minor amount but typically greater than about 0.5 wt %, and preferably from about 1.0 to about 1.5 wt % based on the weight of the oil. The weight ratio of phenols of formula I to formula II generally will range from 1:10 to 10:1 and preferably 1:1 to 1:1.5.
A particularly preferred electrical oil composition comprises a major amount of a paraffinic or naphthenic oil and about 0.1 to 3.0 wt % of 2,6-di-t-butyl phenol and about 0.1 to about 2.0 wt % of 2,6-di-t-butyl cresol in the ratio of about 1:1 to about 1:1.5.
In one embodiment of the invention the composition also includes a tolyltriazole metal deactivator represented by formula III. The preferred metal deactivator is 1, 2, 3 tolyltriazole, which is a reaction product of a benzotriazole and a diphenyl amine. 
wherein R1 and R2 may be the same or different alkyl groups having from about 3 to about 15 and preferably about 4 to about 9 carbon atoms and R3 is an alkyl group of from 1 to about 15 carbon atoms and preferably 1 carbon atom.
The metal deactivator typically will comprise from about 0.01 to about 1.5 wt % based on the weight of the paraffinic oil and preferably from about 0.10 to about 1.0 wt %, most preferably present at about 25% of the total treat of the phenolic compounds.